1. Field of the Invention
The present invention relates to an image signal demodulation apparatus for use in such as a video tape recorder (VTR) and, particularly to an image signal demodulation apparatus capable of reducing moire pattern adequately.
2. Description of the Related Art
A pulse count type demodulator which utilizes zero crossing of an FM image signal as a reference, has been used generally as an FM image signal demodulation apparatus for a VTR. In case of "low" carrier freqeuncy FM signal system in which the carrier frequency f.sub.c of the FM image signal is comparably close to its modulating signal frequency f.sub.p, sideband components of the FM image signal are mixed, upon demodulation, into a baseband of the demodulated signal at an FM demodulator of pulse count type, resulting in signal degradation called moire interference.
According to an exemplary method for dealing with such phenomenon, moire components including a component of 2f.sub.c -2f.sub.p which is produced in the pulse counter are attenuated by delaying a first lower sideband signal with respect to the carrier by a predetermined amount of time which may be about 50 n sec when a carrier frequency f.sub.c of the FM signal is 7 MHz, and a modulation frequency f.sub.p is 5 MHz, and advancing a first upper sideband signal with respect to the carrier by a predetermined amount of time which may be about 15 n sec under the same conditions. Alternatively, the carrier frequency f.sub.c may be set to a value which is 1.5 times the maximum frequency of the baseband signal frequency range such that the moire component becomes out of the baseband range.
On the other hand, in a VTR of such as the S-VHS system (one of the Industry Standard Recording/Playback Systems), the "low" carrier frequency FM recording system mentioned above is employed, there have two sources of degradation of image quality of luminance signal as follows:
[1] As shown in FIG. 1, second and subsequent lower sidebands of an FM signal are frequency-folded with respect to the frequency of 0 Hz generating aliases which interfere in the FM spectrum, resulting a moire interference (hereinafter simply called "moire") after the demodulation. Particularly, when aliases of a first and the second lower sidebands are close in frequency to each other, there may be a beat produced, resulting in conspicuous image quality degradation. A relation between the carrier frequency f.sub.c and the modulating signal frequency f.sub.p in this case can be approximated from f.sub.c -f.sub.p =-(f.sub.c -2f.sub.p) by equation (1). EQU f.sub.p =2f.sub.c /3 (1)
Frequency f.sub.m of the moire after demodulation due to the aliasing of the second lower sideband becomes as follows: EQU f.sub.m =2f.sub.c -2f.sub.p ( 2)
[2] FIG. 2 is a block diagram of a conventional VTR playback system including a conventional image signal demodulation apparatus. An FM signal reproduced from a magnetic tape by a recording/playback head which is not shown, is amplified by a preamplifier 2, and, after its amplitude characteristic and group delay characteristic around the carrier frequency are equalized by a playback equalizer 3, the reproduced FM signal is supplied to an FM demodulator 5 through a limiter 4. When this FM demodulator 5 takes in the form of the pulse counter as shown in FIG. 3, an output signal spectrum becomes as shown in FIG. 4 and frequency components beyond the baseband are attenuated or removed by a low pass filter (LPF) 6. The limiter 4 functions to remove amplitude variation components and to restore the upper sideband. In FIG. 3, reference numerals 8 and 9 indicate a delay circuit and an exclusive OR circuit, respectively. The exclusive OR circuit may be replaced with a multiplier.
As shown in FIG. 4, the output signal contains a component whose frequency is twice the carrier frequency f.sub.c and its lower sidebands 2f.sub.c -2f.sub.p, 2f.sub.c -3f.sub.p, etc., are mixed in the baseband, causing moire. Particularly, when the frequency of the second lower sideband component "f.sub.m =2f.sub.c -2f.sub.p " of the doubled carrier frequency, is close to the baseband signal frequency f.sub.p, a beat is generated, resulting in substantial degradation of image quality. A relation between f.sub.c and f.sub.p, in this case, can be approximated by: EQU f.sub.p =2f.sub.c /3 (3)
because the moire frequency f.sub.m is substantially equal to f.sub.p. From the equations of 2f.sub.c -2f.sub.p =f.sub.m and f.sub.p =2f.sub.c /3, frequencies of major moire components in the above mentioned items [1] and [2] are identical to each other.
The above described conventional image signal demodulation apparatus has the following drawbacks:
(1) It requires two processes (adjustments), that is, the process for increasing the group delay time of the lower sideband with respect to the carrier by a predetermined amount and the process for decreasing the group delay time of the upper sideband with respect to the carrier by a predetermined amount.
(2) In VTRs etc., in order to improve the pulse characteristics of the signal after it is demodulated, it is necessary to equalize the group delay characteristics around the carrier frequency. However, it is difficult to do this together with controlling the group delays of the upper and lower sidebands.
(3) It is impossible to cancel moire due to the aliasing of lower sidebands generated upon modulation, although the moire produced upon demodulation can be attenuated.